Redhorn was where I farmed for mine before eventually unlocking KoD3 at GR19. the exact location I haven't really noted, but it is beyond entering the enemy base. Actually, upon entering the main facility, keep going straight and take the elevator down and you will reach a level with monsters and either a tier 3 fire dragon or earth dragon (sometimes both). What also makes it easier is that you can take the elevator back up afterwards and go back to that save point that you passed along the way, enter and exit Geonet to get the dragons to respawn.

Alright, I'm steadily moving up and now I need Destroyer Outfit and Destroyer Outfit+. I'm guessing I can kill destroyers in the Desert Free Monster Quest. Targeting the Turret? Or Van Haven Waste somewhere. Not entirely sure....???